1. Field of the Invention
The present invention relates to a method and apparatus for the measurement of atmospheric air temperature; and, more specifically, to passive, remote atmospheric air temperature sensing.
2. Description of the Related Art
Atmospheric air temperature sensing and measurements are critical to the performance and safety of aircraft. As performance requirements of modern aircraft increase, the need for more accurate air temperature sensing, with more resolution and higher data gathering rates, becomes more critical. The temperature of the air through which the aircraft is moving impacts its aerodynamics, engine performance, and the environmental control.
Air temperature is also important to flight safety. There are numerous atmospheric disturbances and flight hazards associated with different types of temperature conditions. These include microbursts, clear air turbulence, and icing. Therefore, more accurate and/or faster air temperature measurements, especially when detecting the air temperature ahead of modern aircraft, is important. Warning of temperature variations and clear air turbulence for supersonic aircraft, such as the proposed High Speed Civil Transport, is particularly critical. Temperature variations and clear air turbulence can cause, among other problems, a phenomenon known as xe2x80x9cengine unstart,xe2x80x9d a momentarily loss of engine power. The air temperature must therefore be measured at some distance in front of the aircraft, thereby potentially providing a warning of clear air turbulence and temperature variations and allowing the engines to be adjusted to prevent the unstart occurrence. At supersonic speeds this is difficult.
Air temperature measurements must also be accurate enough to yield good thermal profiles at supersonic speeds which then can be used to timely detect rapid thermal anomalies indicative of clear air turbulence. However, this is difficult to do with traditional technology, such as a thermistor in the airstream, since the aereodynamic heating from the airflow heats the probe above the actual air temperature by a few degrees Celsius for low speed aircraft and up to hundreds of degrees for supersonic aircraft. Therefore, small variations in air craft velocity at supersonic speeds causes atmospheric temperatures measurements to vary greatly.
Beyond the data requirements of modern aircraft, there are also new configuration requirements which impact the fundamental measurement problem. An example is military aircraft having low radar profiles, known as stealth aircraft. This configuration requires that the temperature probe also have a low radar profile. The stealth requirement, thus, cannot be met by traditional physical probes which must extend out beyond the boundary layer, ahead of the aircraft, to sample air undisturbed by the aircraft.
In order to solve the problems associated with conventional immersion type thermometers which physically extend into the air, remote sensors have been developed. They are basically of two types: active and passive. Active systems send out, or emit, a signal that is then, for example, reflected to a detector. Optical systems, such as laser-based (lidar) systems are examples of active systems. These systems, however, suffer from many problems, two of which are particularly significant. First, because they are active, signal power requirements are substantial. Second, and most significantly, active systems are traceable by optical detection systems destroying the stealth aspect of their use.
Passive systems do not emit electromagnetic radiation signals, but register naturally occurring emissions from, for example, heated gas molecules. Since, the temperature of the air, as a gas, is related to the storage of energy by molecules in the gas, as the temperature increases, the radiation emitted by gas molecules also increases. Thus, for example, if one picks nitrogen, carbon dioxide or oxygen, which are three of the most prevalent gas molecules in air, and measures the increase and decrease of radiation by those molecules versus the directly measured temperature of a known reference body, a correlation can be established. By measuring the radiation with a passive detector proximate the aircraft""s skin, one can then determine the temperature of the air through which the craft is passing.
xe2x80x9cRadiometersxe2x80x9d are passive instruments which measure the magnitude of radiation at various wavelengths by passive radiant gas thermometry. Broad spectrum radiometers are able to measure radiation over a number of wavelengths, while narrow spectrum radiometers focus on a very narrow range and maybe even a particular wavelength from a particular element or compound. Airborne, remote sensing radiometers, for the determination of atmospheric temperature from aircraft, hold great promise as the instrument of choice for highly accurate, high speed, stealth atmospheric temperature measurement.
Passive radiant gas thermometry holds several important advantages over conventional airborne thermometer techniques. Radiometry based air temperature measurement starts with the passive collection of radiated emissions from the atmosphere proximate the aircraft. The system can xe2x80x9cseexe2x80x9d beyond the thermal boundary layer and measure infrared radiation some distance in front of the aircraft. The intensity of this radiation is measured at selected wavelengths which are correlated with the absorption bands of particular gases in the atmosphere. For example, U.S. Pat. No. 4,394,575, issued to Nelson, describes a radiometer which measures infrared emissions from the atmosphere at 4.3 xcexcm, which are centered on an absorption band of atmospheric carbon dioxide (C02). The measured intensity of the selected wavelengths of infrared radiation from the atmosphere is then calibrated against radiation emitted from a xe2x80x9cblackbodyxe2x80x9d source having a known temperature and emissivity, in order to calculate the static air temperature. Here the static air temperature is taken to be the true air temperature undisturbed by the presence of any aircraft.
Radiometry based air temperature measurements have several proven advantages over conventional airborne thermometer techniques. First, passive remote sensing radiometers are not adversely affected by aerodynamic heating. Thus, no corrections for aircraft velocity, aircraft attitude (sideslip or yaw) or atmospheric pressure are required. In this manner radiometers provide a xe2x80x9cdirectxe2x80x9d measurement of the air temperature. Second, radiometers are not affected by convective cooling resulting from sensor wetting. In hydrometeor clouds, conventional thermometers can erroneously read several degrees Celsius below the actual atmospheric temperature, due to the sensor probe becoming wet. Third, radiometers can accurately measure the static air temperature beyond the aircraft thermal boundary layer. Such layers can be very thick (several meters) for high speed aircraft, precluding the use of a conventional thermistor entirely. Radiometers are also not adversely affected by shock fronts associated with supersonic flight. Fourth, radiometers typically receive atmospheric thermal radiation through a small optical window normally mounted along the aircraft outer mold-line. An atmosphere-to-sensor interface conformal to the aircraft skin is advantageous to high-speed, high performance and low radar cross-section aircraft. Since the window is optically very transparent (low absorption), window heating from the surrounding skin and atmosphere does not adversely affect the radiometrically measured air temperature.
Radiometry based temperature measurement is ideally suited for high-speed, high-altitude aircraft. Fast and accurate air temperature information are used by these advanced aircraft for optimizing engine efficiency and calculating flight parameters, as well as for warning of approaching atmospheric conditions. Traditional temperature measurement technology, such as a thermistor that extends from the aircraft into the airstream, are inaccurate when the aerodynamic heating from the very rapid airflow warms the probe significantly above the actual air temperature. This is partially offset through pre-flight calibration by introducing a velocity dependent correction term and by use of sophisticated housings. Radiometry technology, unaffected by the aerodynamic heating at supersonic speeds, provides static air temperature information that is very difficult to obtain by traditional temperature measurement technology.
Passive radiometry-based air temperature measurement also has key advantages for military xe2x80x9cstealthxe2x80x9d aircraft. Stealthy aircraft are designed to have minimum radar cross-section and overall low observability. These include all forms of aircraft, from bombers to fighters to helicopters to unmanned air vehicles. Passive radiometers provide these aircraft with the temperature information they need to fly at optimum performance and are fully contained within the body of the aircraft. Their window can be conformal to the aircraft and coated to minimize its radar cross-section. No radiation is emitted from the radiometer, in contrast to laser radars (lidars). Optical countermeasures such as high peak power laser pulses cannot be used to defeat the radiometer. Passive radiometers only accept radiation in very narrow wavelength regions where the atmosphere is strongly absorbing and therefore are nontransmissive to lasers.
Air temperature measurements by radiometry have also an advantage over traditional methods because of their higher accuracy and shorter measurement time. Traditional probes have typical temperature accuracies of 1xc2x0 C. Their measurement time is set by the probe""s thermal inertia, which can be as long as one second or more. Radiometers have demonstrated accuracies of better than 0.1xc2x0 C., even at shorter measurement times. Improved accuracy and as well as the rate at which the samples are taken can help improve estimates of aircraft performance and control. For example, higher accuracy temperature measurements contribute to higher accuracy Mach number measurements which are important to fuel efficiency calculations and in setting engine operating parameters. An example for flight control is that higher accuracy measurements of air temperature contribute to higher accuracy estimates of air density which is used by some flight control systems in the auto-pilot to give an altitude hold mode to alleviate pilot workload.
Although radiometers are the instruments of choice for atmospheric temperature determination, current systems suffer some significant operational draw backs. First, current radiometry technology is too bulky for widespread use in commercial and military aircraft. Current radiometers require a bulky, blackbody reference source to provide infrared reference radiation at a known temperature. This source is required as a reference with which the atmospheric radiation measurement is compared. The maintenance of this reference, at a constant temperature is critical to the accuracy of the measurement. The very bulky thermal insulation, required to maintain the source at a constant temperature, is difficult to miniaturize.
Second, flight hardware must be very reliable. A radiometer requires the infrared source to be switched between radiation from this blackbody source (the reference source) and the atmosphere (the sample source) at rather high frequencies to continually and rapidly make a comparison between the two. This is necessary to generate accurate calibrated air temperature measurements. The switching is typically accomplished by a chopper wheel modulator which allows the infrared radiation viewed by the detector to alternate between the radiation emitted from the outside air and the radiation emitted from the blackbody source. The chopper wheel must rotate at a high rate of speed to minimize the 1/f noise in the detection channel, as well as generate data continuously while the aircraft is moving at a high rate of speed. This places a tremendous amount of stress on the chopper wheel bearing assembly. In flight failure of the bearing causes failure of the system. Thus, expensive redundancy or heavy duty assemblies are required.
Third, prior art radiometers are energy inefficient, which generates heat that can cause the constant temperature of the blackbody reference source to fluctuate. The chopper motor used to rotate the wheel generates significant heat which interferes with the infrared radiation measurement from the source and the reference as well as causing temperature fluctuations in the constant temperature, reference source.
Finally, previous radiometers are subject to system noise interference. 1/f noise is common in optical detectors. It is minimized by modulating or switching the optical signal at a higher frequency and electronically filtering the detected signal at the same frequency. The lower the switching frequency the higher the noise. When this switching is accomplished by a mechanical chopper wheel, the alternation between the radiation emitted from the outside air and the radiation emitted from the ablackbody source, is at a lower, and therefore more noisy, frequency.
It would, therefore, be advantageous to have a passive, remote sensing radiometer for measurement of atmospheric air temperature that was small in size and had low power requirements; had increased the reliability of the in-flight radiometric instrumentation; would provide accurate, direct measurement of the true air temperature, but was not adversely affected by aircraft velocity, aircraft altitude, local pressure/altitude, or cloud wetting; was capable of measuring beyond the aircraft boundary layer and supersonic shock fronts; had fast response measurement which is not limited by the thermal mass of a temperature probe and housing; and, provided multi-wavelength capability for developing range-resolved thermal profiles for application to detecting and warning of engine unstart and clear air turbulence while minimizing sources of noise in the system which limit performance, including the detector 1/f noise and the thermal radiance from the walls. In addition, it would be advantageous to have such a system which was less bulky, more energy efficient and had accuracy at greater speeds and higher altitudes.
A passive, remote sensing device for measurement of atmospheric air temperature that is capable of measuring beyond the aircraft boundary layer and supersonic shock fronts; has fast response, with measurements not limited by the thermal mass of a temperature probe and housing; and provides accurate, direct measurement of the true air temperature, but is not adversely affected by aircraft velocity, aircraft altitude, local pressure/altitude, or cloud wetting has been discovered. The inventive device is small in size while minimizing sources of noise in the system, has low power requirements and increased reliability, as well as providing multiwavelength capability for developing range-resolved thermal profiles for application in detecting and warning of engine unstart and clear air turbulence.
According to the invention, a passive, remote sensing device for measurement of atmospheric air temperature, which is less bulky and has accuracy at greater speeds and higher altitudes and can make in-flight passive spectroscopic measurements of air temperature at select distances beyond an aircraft thermal boundary layer, uses an actuated movable grating modulator for switching the detected infrared radiation between the atmosphere and a reference source. The actuated, movable grating modulator, which is preferably electrostatically actuated, is more energy efficient and more reliable than a conventional chopper wheel. The device of the present invention includes microelectronic machine based parts that are less than {fraction (1/1000)} the size of comparable macro-sized parts and allows the miniaturization of the device such that the entirety of the device can be maintained at constant temperature, eliminating bulkiness. Optical filter selection can be adjusted to provide a very localized temperature measurement or a longer-path measurement (such as ahead of the aircraft). The device is able to both measure radiation over a number of wavelengths, as well as a narrow spectrum focused on a very narrow range including a particular wavelength.
According to the broad aspect, a device for passive, remote sensing of air temperature employs an actuated modulator for periodically alternating the radiation for detection by an infrared radiation detector between the infrared radiation emitted from the atmosphere and that emitted from an infrared reference radiation source such that the radiation emitted from the atmosphere is compared to that emitted by the reference source at a given infrared wavelength in order to determine the air temperature. In one aspect, a device for passive, remote sensing of air temperature which has a window for allowing infrared radiation to pass from the atmosphere into the device and onto a detector which measures radiation intensity at a selected infrared wavelength; and a temperature reference material that provides infrared reference radiation from a surface of known emissivity and temperature comprises an actuated modulator for periodically alternating the source of radiation for detection by an infrared radiation detector between the infrared radiation emitted from the atmosphere and that emitted from the infrared reference radiation source such that the radiation emitted from the atmosphere is compared to that emitted by the reference source at a given infrared wavelength in order to determine the air temperature.
The modulator of the present invention can be electrostatically, thermally, or piezoelectrically actuated. Preferably, the modulator comprises an electrostatically actuated, vertically movable grating positioned proximate a fixed base, wherein the vertical position of the movable grating with respect to the fixed base is adjusted by applying an electric potential (voltage) between the movable grating and the base. A vertically moving grating includes an electrostatically movable first grating spaced above a fixed second grating. The gap between the gratings can be adjusted from a wide position to a narrow position by applying a preset electric potential across the gratings. In one preferred aspect, when the gratings are in the wide gap position, the interference between light reflected from the first and second gratings is entirely constructive, and almost all the light is reflected off the pair of gratings as 0th order diffracted light. In contrast, when the gratings are in the narrow gap position, the interference between light reflected from the top and bottom gratings is destructive, the 0th order reflection drops to almost zero intensity, and most of the light energy is transferred into 1st order satellite bands.
In accordance with the invention, 0th order light and 1st order light are diffracted in different directions by the modulator. Thus, if the modulator and detector are aligned such that the 0th order light hits the detector, then the 1st order light will be diffracted away from the detector.
In a preferred aspect of the present invention, this phenomenon of diffracted light is exploited by aligning the modulator such that the detector alternates between receiving 0th order light from atmospheric thermal emissions and 1st order light from the temperature reference material.
According to the invention, a wide range of prior art modulators or xe2x80x9cswitchesxe2x80x9d can be used in the system. These include thermal or electrostatic actuators. Examples of these modulators include: variable blaze gratings, variable transmissive gratings, micro-chopper wheels, micro-shutters, reflective or transmissive Fabry-Perot devices, and movable mirrors. The preferred embodiment has the advantages of being electrostatically driven, producing large angular displacements between the reference and main signal beams, being simple in construction, working over a broad range of wavelengths, producing the necessary modulation frequencies, and requiring only low operating voltages.
The window or opening for allowing radiation from the atmosphere to pass into the device is made from any suitable material designed for such use that is transparent to the wavelength of radiation that is to be measured by the device. Such materials that can be formed into the mold line of an aircraft are greatly preferred. The window can include a solar blind optical filter which passes select portions of the infrared spectrum and blocks wavelengths in the visible spectrum. In another aspect, the window is coated with a hard material such as a diamond-like-carbon in order to retard scratching or abrasion. In this preferred aspect, the window that allows thermal radiation from the air to enter the radiometer and the temperature reference material are arranged such that the window is in the path of the 0th order radiation diffracted by the modulator, and the temperature reference material is in the path of 1st order radiation. In other words, the detector alternately views images of the window and the temperature reference material which are diffracted off of the modulator. The modulator permits the radiometer to alternate between measurements of infrared radiation emitted by the atmosphere and infrared radiation emitted by the reference material at frequencies exceeding 500 Hz. This high frequency sampling provides fast, statistically averaged air temperature information, and virtually eliminates the 1/f noise in the sample and reference signals.
The temperature reference material can comprise any conventional black-body calibration source. For example, conventional blackbody sources made by Epply Laboratory, Inc., such as the Eplab Model BB25TB, are adequate conventional sources for the present invention. In one aspect, the bulky, blackbody reference source is replaced with a thin layer of temperature reference material that can be coated onto a wall of the radiometer.
The temperature of the temperature reference material is preferably measured with a temperature measurement means. In a preferred aspect, the temperature measurement means comprises a resistance temperature detector. However, a thermocouple, thermistor, or similar temperature transducer can be used.
The device of the present invention is preferably kept at a constant temperature with a temperature control means. Preferably, the temperature control means is selected from the group consisting of a thermoelectric cooler, a Dewar, and a cryo-cooler. In one embodiment, the temperature control means is a thermoelectric cooler which can for example, maintain the radiometer at a constant temperature between about 160 K and about 293 K. In another embodiment, the temperature control means is a liquid nitrogen cooler, which can maintain the radiometer at a constant 77 K. In another embodiment, the temperature control means is a liquid filled Dewar. In another preferred aspect of the present invention, focusing means are used to focus the filtered light from the atmosphere onto the modulator.
Focusing means can also be used to focus the light emitted by the temperature reference material onto the modulator, and the light diffracted by the modulator onto the detector. The focusing means preferably comprise lens optics that are coated with anti-reflection coatings to maximize light transmission through the optics, and minimize scattered light. Reflective, diffractive or a combination (hybrid) optic focusing means, well known in the art, can also be used for focusing.
The wavelength filtering means of the present invention can be dispersive or non-dispersive. Preferred filtering means include interference filters, gas correlation filters, and continuously adjustable wavelength selective filtering means. The adjustable wavelength selective filters can include wavelength selective elements like diffraction gratings, prisms, and etalons. In a preferred aspect of the present invention the filtering means comprises a Fabry-Perot filter which can continuously scan across a wide wavelength range, while maintaining a very narrow bandwidth around the central wavelength of interest.
The detector of the present invention is preferably selected from the group of detectors consisting of HgCdTe detectors, PbSe detectors, PbS detectors, Si detectors, Ge detectors, pyroelectric detectors, Golay Cells, and InAs detectors. Quantum Well Infrared Photodetectors (QWIPs detectors) and microbolometers are also contemplated as light detector technologies that can be used to make the detector for the present invention.
A method of determining airborne air temperature measurements with a remote, passive device is also disclosed. In accordance with this method invention, remote, passive device measures the intensity of light from the atmosphere at select wavelengths, preferably wavelengths between 1 xcexcm and 20 xcexcm where the blackbody emission spectrum peaks for typical atmospheric temperatures, and more preferably at 4.3 xcexcm or 15 xcexcm which are carbon dioxide absorption bands, and compares the measured intensity of this light with light intensity from a temperature reference material at a known temperature, in order to find the temperature of the atmosphere by alternating between measurements of radiation intensity of the atmosphere, and the temperature reference material using an actuated movable grating modulator.
In another preferred aspect of the method of the present invention, a tunable wavelength filtering means (e.g., a Fabry-Perot filter) is used to map air temperature for a volume of air surrounding an aircraft. This method involves scanning the wavelength of the atmospheric thermal radiance around a center wavelength that is associated with an absorption band for an atmospheric gas. When the radiometer bandpass is centered on the absorption band, the only radiance which reaches the detector is that which came from a short range in front of the instrument, because radiance from further out has been absorbed by the intervening gas. Conversely, when the bandpass is to the side of the absorption band center, the nearby gas is less effective as an absorber and radiance from longer distances will be detected. The radiometer essentially performs an exponential average over a range determined by the absorption coefficient of the gas at a given wavelength. By operating at multiple wavelengths, each averaging over a progressively longer range, it is possible to determine the radiance, and hence the temperature, as a function of distance in front of the radiometer. The choice of a set of center wavelengths and bandwidths help to define a set of xe2x80x9cweighting functionsxe2x80x9d used in an iterative process to convert the measurements of radiance into temperature profiles.
In an alternate preferred embodiment of this method, multiple discrete spectral bands can be investigated without the need for a scanning wavelength filter. As with the scanning implementation, spectral bands are selected to provide temperature measurements at distinct range bins from the instrument. Such spectral bands can be selected by numerous optical configurations. These include multiple detectors used in conjunction with dispersive optical elements, like diffraction gratings and prisms; multiple detectors used in conjunction with multiple interference filters; multiple detectors used in conjunction with tunable dispersive optical elements like diffraction gratings and prisms; sandwiched detectors used in conjunction with tunable dispersive optical elements like diffraction gratings and prisms; and singular detectors used in conjunction with tunable dispersive optical elements like diffraction gratings and prisms.
In an alternate preferred embodiment of this method, the air temperature can be mapped with a non-scanning wavelength filter (e.g., an interference filter) by vertically scanning the field of view of the radiometer. Algorithms allow a temperature versus altitude profile to be derived.
The preferred radiometer of the present invention comprises: a window for allowing radiation from the air to pass into the radiometer; a temperature reference material that emits radiation with an intensity and spectrum determined by Plancks law; a wavelength filtering means for allowing only selected wavelengths of infrared radiation from the atmosphere and the temperature reference material to impinge on a detector which measures radiation intensity; an electrostatically actuated movable grating modulator for periodically alternating radiation measured by the detector between the radiation from the air and the well defined reference radiation from the temperature reference material, wherein the modulator comprises an electrostatically actuated vertically movable grating positioned above a fixed base, and the vertical position of the movable grating with respect to the fixed base is adjusted by applying a voltage between the movable grating and the base.
Alternatively, a preferred aspect of the present invention replaces a conventional black-body source with a thin layer of reference material that is attached to a surface at the radiometer. In this preferred aspect, reference materials include Martin Black, Enhanced Martin Black, Cat-A-Lac paint, dendritic platinum, and materials formed by anodizing a surface in the radiometer. Any such coating with a known emissivity at the operating wavelength(s) could be used. However ones with emissivity near 1 are preferred.
As a means of enhancing the radiometer""s calibration and accuracy and as a means of assessing the proper function of the radiometer, thermal emission from the detector can be re-directed back toward, and collected by, the detector itself. Since the detector temperature is controlled and stabilized, re-collecting the detector""s thermal emission provides a means of calibrating the radiometers electronic output to a known temperature.
This method can be used with and without the said blackbody calibration source. When this method is used in conjunction with the blackbody calibration source, both the final electronic gain and the electronic offset can be determined, improving system accuracy with an improved calibration methodology. Alternately, the detectors thermal emission can be used to entirely replace the blackbody calibration source. Finally, collecting the detector""s thermal emission provides information on the functionality of various electronic subsystems. Such information can be used to determine the health status of these subsystems. There are different ways to allow the detector to view its own radiance. The preferred method is to mount a mirror or corner cube reflector to the wall and have the modulator alternate the viewing direction of the detector from the air, to the reference material, to the mirror.
The thermal characteristics of the section of the wall which acts as the reference is important because it serves to calibrate the air temperature measurements. The thermal characteristics of the sections of the wall not making up the reference are also important because their thermal emission can scatter off of the optics into the detector giving a background radiation above which the signal must be detected. If this radiation is not stable then it causes systematic errors in the air temperature measurement. Depending on the exact configuration, these sections of the walls can be coated with low emissivity materials (like gold) or high emissivity materials like blackbody coatings. More exotic materials like photonic bandgap materials which could virtually eliminate thermal emission at the operating wavelengths of the radiometer could also be used.
In another preferred aspect of the present invention, focusing means are used to focus the filtered radiation from the atmosphere onto the modulator. Focusing means can also be used to focus the radiation emitted by the temperature reference material onto the modulator, and the radiation diffracted by the modulator onto the detector. The focusing means preferably comprise lens optics that are coated with anti-reflection coatings to maximize radiation transmission through the optics, and minimize scattered radiation.
All or part of the radiometer of the present invention is preferably kept at a constant temperature with a temperature control means. In one preferred aspect, the temperature control means is a thermoelectric cooler which can maintain the key radiometer components at a constant temperature between about 160 K and about 293 K. In another preferred aspect, the MEMS radiometer is contained within the detectors cryo-cooler or a cryogenic liquid filled Dewar which maintains the radiometer at a constant temperature near 77 K. In another preferred aspect, a thermoelectric cooler is used to cool the radiometer, but an electronic control loop is used to keep the radiometer temperature the same as the measured external temperature in order to reduce aspects of the electronic noise. A Dewar can be used to maintain temperature below ambient air tempeture.
The radiometer of the present invention can also be made from microelectromechanical system (MEMS) based components. In this preferred aspect, radiometry components including optics, windows, wavelength filtering means, and the electrostatically actuated movable grating modulator are micro-machined to sizes ranging from 1-1000 microns, using techniques originally developed in the semiconductor chip fabrication industry. In another preferred aspect, the MEMS based radiometer is built on a single silicon wafer chip. The small size of a MEMS radiometer increases the ability to control the radiometers temperature and therefore reduces the effect of stray radiance on the temperature measurement.
The MEMS based radiometers, built with batch processing techniques, are significantly less costly than macro-sized radiometers. Moreover, these radiometers require significantly less power than macro-sized radiometers, making MEMS-based radiometers much more practical for airborne air temperature measurements.